Recently, with a rapid increase in mobile traffic, frequency bands used in a mobile network are increasing. Therefore, for a filter circuit mounted on a communication device, a function of selecting and suppressing a plurality of signals having frequencies different from each other is demanded. Further, in order to improve interference resistance performance, various types of circuits such as a low noise amplifier (LNA) desirably have a differential configuration, and a balun (balanced-to-unbalanced transformation) circuit may be disposed in a subsequent stage of a bandpass filter. It is known that a bandpass filter circuit and a balun circuit may be configured as a balun bandpass filter circuit including functions of both circuits, and similarly to a filter circuit, the balun bandpass filter circuit also needs to respond to a plurality of frequency bands.
As a balun bandpass filter circuit that responds to a plurality of frequency bands, a balun bandpass filter circuit in which a transmission line such as a micro-strip line is configured on a planar circuit is known. PTL 1 and NPL 1, for example, describe a balun bandpass filter circuit including a split-ring resonator and describe that a resonance frequency may be changed by a variable capacitance loaded on the split-ring resonator. Further, NPL 1 discloses a dual-band balun bandpass filter circuit, having two frequencies as a passband, including a micro-strip coupling line.
PTL 1, NPL 1, and NPL 2 describe a technique relating to a filter circuit as a related technique.